When evaluating whether a given earth formation contains valuable materials, such as hydrocarbons, a core sample from the earth formation may be procured. When the core sample is returned to the surface, the core sample, any fluids entrapped within the core sample, and any fluids that escaped the core sample but were captured by the coring tool may be analyzed to determine the characteristics exhibited by the earth formation. To ensure that the coring tool more accurately represents the actual characteristics of an earth formation at the end of a borehole, steps are taken to reduce the likelihood that contaminants enter a receptacle that is to receive the core sample. For example, an entrance to the receptacle may be sealed shut while advancing the coring tool into the borehole to reduce the likelihood that materials other than the core sample (e.g., drilling fluid and particles suspended within the drilling fluid) enter the receptacle and contaminate the receptacle, the core sample, or any other material in the receptacle. The entrance to the receptacle may be sealed shut by, for example, an activation module that is intended to block the entrance to the receptacle while the coring tool is advanced into the borehole and to unblock the entrance to the receptacle as a core sample is introduced into the receptacle. As a further example, the receptacle may be substantially emptied of material and then filled, and potentially pressurized, with a presaturation fluid (i.e., a fluid of known composition that will not contaminate the core sample) before the coring tool is introduced into the borehole. The presaturation fluid may be a fluid that is not wettable to a sponge material lining the interior of the receptacle, the sponge material being wettable to a fluid of interest expected to be found within the core sample, such as oil.